Wireless charging systems allow powering and charging of portable devices by using energy from electrical or magnetic fields at proximity to the portable devices instead of providing power via a wired connection. Wireless charging may be used in various applications including electric tooth brushes, mobile phones, televisions, cars, etc. In these applications, typically a single transmitter (Tx) is used to charge a single receiver (Rx). Such a one-to-one relationship may be imposed due to limitations of magnetic induction (MI) technology. For example, a multi-device charging mat that uses MI technology includes several transmitters to support several receivers of devices that are placed on the mat, so that a one-to-one relationship between transmitters and receivers is maintained.
Wireless charging using magnetic resonance (MR) technology enables charging of multiple Rx devices to be charged simultaneously positioned in proximity to the Tx. MR charging power is affected by distance between the coils of the Tx and Rx and by coil geometry. For simplicity, “distance” is used herein to refer to both distance and geometry affecting wireless power transfer efficiency. MR charging may also be affected by cross-device effects. In particular, the device Rx of a first device that is closer to the Tx will draw more power from the MR field, leaving less power for a second device that is at a larger distance from the Tx than a first device. Hence, a charging time of the second device may be significantly longer than of the first device. Uneven charging times between the first device and the second device may impact a user experience. For example, a user may be frustrated to find that the second device is barely charged when the first device has already been fully charged.